Copolymerization is one of the most widely used techniques in the polymer industries. For example, polystyrene is one of the largest volume thermoplastic resins in commercial production today. However, homopolymer polystyrene resin is typically a brittle resin having a poor impact strength, and is only suited to applications where its brittleness is acceptable. It has long been known that the impact strength of polystyrene can be greatly improved by the blending/grafting of rubbery particles dispersed throughout the polystyrene matrix. Polystyrene resins of improved strength achieved by incorporation of rubbery particles are often referred to as high impact polystyrene (HIPS).
The physical characteristics and mechanical properties of HIPS are dependent upon many factors, including rubber dispersion and rubber morphology. For example, when HIPS polymers are formed by incorporation of polybutadiene (PB) phase in polystyrene (PS) through the in situ formation of PB-graft-PS, phase separation begins early in the polymerization because of the immiscibility of the rubber within the polystyrene being formed and the depletion of the styrene phase. The immiscibility of PB in PS and the in situ formation of the compatibilizer, PB-graft-PS, gives rise to the formation of varied morphologies of the lamellar, globular, capsule, and other types. These microstructures are responsible for the absorption of energy when the material is subjected to high intensity (impact) or low intensity (tension) forces.
Copolymerization of styrene and butadiene is also necessary in preparing other rubber modified styrenic polymers such as high impact polystyrene, styrene-maleic anhydride copolymer, methylmethacrylate-butadiene-styrene (MBS) copolymer, transparent impact polystyrene, and acrylonitrile butadiene styrene copolymer (ABS). ABS resin generally comprises a rigid matrix copolymer phase of styrene and acrylonitrile monomers having dispersed therein a graft copolymer of a butadiene rubber substrate grafted with the styrene/acrylonitrile copolymer. ABS resins are most often produced by initially preparing a polybutadiene seed latex and copolymerizing styrene and acrylonitrile in emulsion in the presence of the seed latex.
The copolymerization of styrene and polybutadiene to produce HIPS, ABS, and MBS can be conducted via free radical polymerization between the styrene and polybutadiene. Free radical polymerization can be initiated by the use of an initiator. The free radical polymerization is conducted through the unsaturated carbon bonds. Grafting preferentially occurs across the 1,2-vinyl bonds in the polybutadiene. In addition, the location of the 1,2-vinyl bonds in the polybutadiene polymer will determine where grafting takes place. Therefore, it is desirable to be able to control the location of the 1,2-vinyl bonds in the polybutadiene.
Rather than using a free radical polymerization initiator, it is also known to attach radical producing nitroxyl groups to the ends of polymer chains that will produce grafted polymers. However, these nitroxyl groups are subject to thermal decomposition at ˜100° C. This makes it difficult to handle the finished polymer in a production environment involving workup and storage.
Advantageously, the present invention provides a novel method of copolymerization that enables the production of engineered plastic copolymer materials such as HIPS with the ability to control the location of the grafting site during polymerization.